Manual J Load Calculator

Residential heating & cooling load estimation

Simplified Estimation Only

This is a simplified Manual J estimation for preliminary sizing. It does NOT replace a full ACCA Manual J calculation performed by HVAC software. Improper sizing leads to comfort issues, high humidity, short cycling, and equipment failure. Always use professional load calculation software for final equipment selection.

1 Location & Design Conditions

Climate Zone

Outdoor Design Temp (Cooling)

°F

Outdoor Design Temp (Heating)

°F

Indoor Design Temp

°F

2 Building Dimensions

Conditioned Floor Area

ft²

Average Ceiling Height

Number of Stories

Foundation Type

3 Building Envelope

Wall Insulation

Ceiling/Attic Insulation

Wall Construction

4 Windows & Doors

Total Window Area

ft²

Window Type

Solar Heat Gain Coefficient

Primary Window Orientation

Number of Exterior Doors

Door Type

5 Infiltration & Ventilation

Building Tightness

Mechanical Ventilation

6 Internal Loads

Number of Bedrooms

Number of Occupants

Kitchen Appliances

7 Duct System

Duct Location

Duct Insulation

Calculation Results

Cooling Load

BTU/h

Cooling Capacity

Tons (Nominal)

Heating Load

BTU/h

Heating Capacity

kW (Electric) / kBTU/h (Gas)

Cooling Load Breakdown

Walls & Ceiling: -

Windows (Conduction): -

Windows (Solar): -

Doors: -

Infiltration: -

Internal Gains (People): -

Internal Gains (Appliances): -

Duct Losses: -

Total Cooling: -

Heating Load Breakdown

Walls & Ceiling: -

Windows: -

Doors: -

Floor/Foundation: -

Infiltration: -

Duct Losses: -

Total Heating: -

Equipment Sizing Recommendations

Air Conditioner / Heat Pump:

Furnace / Heating:

Cooling Load/ft²

BTU/h per ft²

Heating Load/ft²

BTU/h per ft²

Sensible Heat Ratio

0.75

(Estimated)

CFM Required

@ 400 CFM/ton

Important Notes

• This is a simplified estimation. A full Manual J calculation considers many more factors.
• Equipment should be sized at 100-120% of calculated load. Avoid oversizing!
• Heat pump capacity decreases as outdoor temperature drops - consider backup heat.
• For humid climates, slight undersizing helps with dehumidification.
• Always verify with professional HVAC load calculation software (Wrightsoft, CoolCalc, etc.)

Manual J Overview

ACCA Manual J (Residential Load Calculation) is the industry standard for calculating heating and cooling loads for residential buildings. Proper load calculation is essential for equipment sizing - oversized equipment leads to short cycling, poor humidity control, and wasted energy.

Key Factors in Manual J

Building Envelope

Wall, ceiling, floor insulation R-values
Window U-factor and SHGC
Air infiltration rate
Duct location and insulation

Design Conditions

Local outdoor design temperatures (99% heating, 1% cooling)
Indoor design conditions (typically 70-75°F)
Daily temperature range
Humidity levels

Why Proper Sizing Matters

Oversized Equipment

Short cycling
Poor humidity control
Uneven temperatures
Higher energy bills
Premature wear

Properly Sized

Long run times
Even temperatures
Good humidity control
Maximum efficiency
Equipment longevity

Undersized Equipment

Can't maintain setpoint
Runs continuously
Excessive energy use
Comfort complaints
Equipment strain

IECC Climate Zone Reference

Zone	Description	Example Cities	Typical Design Temps
Zone 1	Very Hot/Humid	Miami, Key West, Honolulu	Cool: 91°F / Heat: 50°F
Zone 2	Hot/Humid	Houston, Phoenix, New Orleans	Cool: 97°F / Heat: 30°F
Zone 3	Warm/Mixed-Humid	Atlanta, Dallas, Los Angeles	Cool: 94°F / Heat: 25°F
Zone 4	Mixed	St. Louis, Baltimore, Seattle	Cool: 93°F / Heat: 15°F
Zone 5	Cold	Chicago, Boston, Denver	Cool: 91°F / Heat: 5°F
Zone 6	Cold	Minneapolis, Burlington, Helena	Cool: 89°F / Heat: -10°F
Zone 7	Very Cold	Fargo, Duluth, International Falls	Cool: 86°F / Heat: -20°F